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第六部分 电缆的 EMC 设计

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第六部分 电缆的 EMC 设计. 场在导线中感应的噪声 电缆之间的串扰. 处于电磁场中的电缆. S. h. 电磁场在电缆上的感应电压. 1V/m 场强产生的电压. dBV. 3. 1. 2. 0. -10. h = 0.5m S: A = 100m B = 30m C = 10m D = 3m E = 1m. A. 与 S 、 h 无关. -20. B. C. -30. D. -40. E. -50.

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slide1

第六部分电缆的EMC设计

  • 场在导线中感应的噪声
  • 电缆之间的串扰
slide3
电磁场在电缆上的感应电压

1V/m场强产生的电压

dBV

3

1

2

0

-10

h = 0.5m

S: A = 100m

B = 30m

C = 10m

D = 3m

E = 1m

A

与 S、h 无关

-20

B

C

-30

D

-40

E

-50

10kHz 100kHz 1MHz 10MHz 100MHz 1GHz 10GHz

slide4
平衡电路的抗干扰特性

VD

V1

电磁场

I1

V2

I2

VC

平衡性好坏用共模抑制比表示:

CMRR = 20lg ( VC / VD )

高频时,由于寄生参数的影响,平衡性会降低

slide5
提高共模干扰抑制的方法

平衡电路

平衡电路

共模扼流圈

屏蔽电缆

CMRR

CMRR

f

f

slide7
屏蔽电场

0V

电缆长度 < /20,单点接地

电缆长度 > /20,多点接地

slide8
磁场对电缆的干扰

感应电压

磁通

VN

回路面积A

VN= ( d  / dt ) = A ( dB / dt )

当面积一定时

slide9
减小感应回路的面积

理想同轴线的信号电流与回流等效为在几何上重合,因

此电缆上的回路面积为0,整个回路面积仅有两端的部分

slide10
屏蔽电缆减小磁场影响

VS

VS

VS

只有两端接地的屏蔽层才能 屏蔽磁场

slide11
抑制磁场干扰的试验数据

100

(A)

1M

0

13

100

(D)

1M

27

100

(B)

1M

每米18节

28

100

1M

(E)

1M

13

100

(C)

slide12
抑制磁场干扰的实验数据

100

80

(F)

1M

63

100

(I)

1M

每米18节

55

100

1M

(G)

77

1M

70

100

100

(J)

1M

(H)

slide13
导线之间两种串扰机理

R0

RL

C

M

IL

R2G

R2L

IL

IC

IC

slide14
耦合方式的粗略判断

ZSZL < 3002: 磁场耦合为主

ZSZL > 10002: 电场耦合为主

3002 < ZSZL < 10002:取决于几何结构和频率

slide15
电容耦合模型

C12

C12

V1

VN

C1G

C2G

C2G

R

C1G

V1

R

j  [ C12 / ( C12 + C2G)]

VN =

V1

j  + 1 / R ( C12 + C2G)]

slide16
耦合公式化简

j  [ C12 / ( C12 + C2G)]

VN =

V1

j  + 1 / R ( C12 + C2G)]

R >> 1 / [ j  ( C12 + C2G )]

R << 1 / [ j  ( C12 + C2G )]

VN = V1 [ C12 / ( C12 + C2G ) ]

VN = j R C12 V1

slide17
电容耦合与频率的关系

VN = j RC12V1

C12V1

VN =

(C12 + C2G)

耦合电压

1 / R (C12 + C2G)

频率 

slide18
屏蔽对电容耦合的影响-全屏蔽

C2S

C1s

C1s

V1

C1G

CSG

Vs

C1G

V1

Vs

CsG

屏蔽层不接地:VN = VS =V1 [ C1S / ( C1S + CSG ) ],与无屏蔽相同

屏蔽层接地时:VN = VS = 0, 具有理想的屏蔽效果

slide19
部分屏蔽对电容耦合的效果

C12

C2S

C1s

C1s

C1G

C12

CSG

C2G

V1

VN

VN

V1

CsG

R 很大时:VN = V1 [ C12 / ( C12 + C2G + C2S ) ]

R 很小时:VN = jRC12

slide20
互电感定义与计算

a

回路1

b

回路2

a

定义: 自感L = 1 / I1 , 互感 M = 12 / I1

 1是电流I1在回路1中产生的磁通, 12 是电流I1在回路2中产生的磁通

M = (  / 2  )ln[b2/(b2- a2)]

slide21
电感耦合

I1

R2

I1

R1

M

VN

V1

VN

V1

R2

R

R1

R

VN = d12 / dt = d(MI1)/dt = M dI1 / dt

slide22
电感耦合与电容耦合的判别

电容耦合

IN = j  C12V1

V

R1

R2

电感耦合

VN = j  M12 I1

V

R1

R2

slide23
非磁性屏蔽对电感耦合的影响

I1

M1S

M12

关键看互感是否由于屏蔽措施而发生了改变

slide24
双端接地屏蔽层的分析

I1

导体1

~

V12 = j  M12 I1

VS2 = j  MS2 IS

VN = V12 + VS2

M1S

M12

IS

屏蔽体

MS2

+ -

- +

导体2

V12

VS2

求解这项

slide25
VS2项求解

LS = / ISMS2 = / IS

因此:LS = MS2

+

+

+

+

VS2 = j MS2 I S

= jMS2 ( V S / ZS)

= jLS [ V S / ( jLS+RS )]

= VS [ j / ( j+RS/LS)]

+

+

+

+

+

屏蔽层

导体2

slide26
屏蔽后的耦合电压

VN = V12 + VS2

V12 = j M12I1 VS = j M1SI1

因为:M12 = M1S

所以:VS = j M12I1

所以:VS2 = j M12I1 [ j / ( j+RS / LS)]

V12

VN = V12 - V12[ j / ( j+RS / LS)]

= V12 [ (RS / LS) / ( j+RS / LS)]

slide27
屏蔽层的磁场耦合屏蔽效果

VN

无屏蔽电缆

屏蔽效能

VN= M12 I1(Rs / Ls )

VN= j M12 I1

屏蔽电缆

lg

Rs / Ls

slide28
长线上的耦合电压

短线近似线

耦合电压

驻波区域

低频区域

/10 /4 /2 3/4  lg f